A variety of aqueous coating compositions comprising epoxy resin and acrylic polymer are known in the art and several are commercially available. In order to obtain desirable characteristics for many critical end use applications, such as coatings for sanitary cans, it is considered necessary to include a high proportion of epoxy resin in the composition. Epoxy contents of 60 to 80 percent are commonly used. Typical epoxy acrylic compositions are disclosed in U.S. Pat. No. 4,247,439 to Matthews and Sommerfield; U.S. Pat. Nos. 4,212,781 and 4,308,185 to Evans and Ting; and U.S. Pat. No. 4,302,373 to Steinmetz; all of which are incorporated herein by reference.
U.S. Pat. No. 4,285,847 to Ting discloses epoxy acrylic compositions in which the epoxy acrylic is made by the free-radical grafting of ionizable side chains onto an epoxy backbone; dispersing this product in water and thereafter polymerizing, in situ, addition polymerizable monomers which may or may not also contain ionizable groups. By this means the solids content of the composition is increased and the proportion of total epoxy resin in the composition is reduced by replacement with the cheaper addition polymer which serves as a diluent, thereby reducing the cost of the composition. However, the use of certain monomers in such a method may give a product with an undesired monomer residue. For example, the use of styrene and butyl acrylate in the preparation of the diluent polymer results in a product having an objectionable odor which is attributed to residual butyl acrylate which polymerizes more slowly than styrene. The method of this invention can be used to solve the problem of objectionable odor caused by butyl acrylate but is by no means so limited. The method has broad applicability in cases where an objectionable monomer residue occurs, particularly in aqueous coatings intended for use as sanitary can coatings.